William Hague today suggested the appointment of the British peer Cathy Ashton as Europe's foreign minister was the result of deal that could damage the UK's national interest.

The shadow foreign secretary said French newspapers were reporting that Ashton had got the foreign minister post in return for an assurance that Michel Barnier, a French MEP, would become a vice-president of the European commission, with responsibility for the internal market and financial affairs.

"If Gordon Brown has done a deal that would mean a French commissioner being in charge of the economic issues that affect Britain the most, then that could be a serious concern," Hague said.

"Our French partners have a different view on market issues that touch on Britain's vital economic interests."

Hague based his claim on a report in Le Monde which said Brown struck a deal with the French president, Nicolas Sarkozy, over the jobs.

One Tory official claimed a "stich-up" of this kind was counter to the national interest because "the French have made no secret of their wish to impose new regulation on the City of London".

Hague also sent a letter to David Miliband, the foreign secretary, asking for confirmation of Le Monde's claim.

In it, he wrote: "The financial services sector is an area of extreme sensitivity for the British national interest.

"It is of crucial importance to the future of a pro-growth EU that there is no retreat from the principle of a free market within the EU."

To bolster their claims, the Tories said Barnier had previously criticised the size of the British budget rebate and blamed "too much free market liberalism" for the 2008 global food crisis.

Ashton's appointment was announced at a special summit in Brussels last night, at which another largely unknown figure, the Belgian prime minister, Herman Van Rompuy, became the first EU president.

Earlier today, Ashton acknowledged that there had been other candidates who could have done the job of EU foreign minister – or high representative for foreign and security policy, as the post is officially described.

But she insisted she was ready to join the foreign ministers of the world's most powerful nations at the "top table".

"Over the next few months and years, I aim to show that I am the best person for the job," she told BBC Radio 4's Today programme.

"I think, for quite a few people, they would say that I am the best person for the job and I was chosen because I am, but I absolutely recognise there are a number of candidates around, all of whom would have been extremely good, extremely able.

"I hope that my particular set of skills will show that, in the end, I am the best choice."

She pointed out that, since becoming trade commissioner last year when Peter Mandelson was recalled from Brussels to rejoin the government, she had already taken the lead for the EU in high-level trade talks with China.

But having served for a number of years as a Labour minister in the House of Lords, including as leader of the Lords, she has not had the experience of being directly elected.

"I am humbled by it, in that I am very conscious of those who have been elected, so it's why I spend a lot of time in the European parliament," she said.

"It's why, when I was leader of the House of Lords, I was very conscious of the role of the House of Commons, of MPs and the importance of those elected representatives.

"But while I wasn't being elected, I spent 28 years doing negotiation in all kinds of fora."

Ashton said she had discovered only in the past few days that she was being discussed as a possible candidate for high representative.

The suggestion was taken up at a meeting of the socialist group leaders yesterday, at which Brown finally abandoned his campaign for Tony Blair to become president.

"I was told that there was a great deal of support for me ... and then that translated into further meetings with other prime ministers right across the political spectrum and across Europe, and then finally unanimity," Ashton said.

"They reached a conclusion that, certainly from last night's experience, I think they are very comfortable with."

She said she would draw her authority in international negotiations from the general affairs council of EU foreign ministers, which she will now chair.

"The council will deliberate, will determine the views, with my support, I hope with my input and my expertise, and that will be the voice I will speak with," she said.